1. Field of the Invention
The present invention relates to a superconducting radiometry apparatus for performing an element analysis or an impurity inspection.
2. Description of the Related Art
In a radiometry apparatus used in element analysis, impurity inspection or the like, there is noted a superconducting radiation detection apparatus utilizing a superconductor detector capable of improving by at least one figure, an energy resolving power of a conventional radiometry apparatus which uses a semiconductor detector.
Since the energy resolving power of the radiometry apparatus using the semiconductor detector depends on an energy gap width that a semiconductor has, it is impossible to become below 130 eV. On the other hand, among the superconducting radiation detection apparatuses using the superconductor detector, there is especially expected a micro-calorie meter capable of making the energy resolving power into 10 eV or less.
The micro-calorie meter is constituted by an absorber absorbing an X-ray, a thermometer whose resistance value changes under a constant voltage by a heat generated by the absorber, and a membrane for controlling a heat flow rate by which a heat generated by the absorber and the thermometer escapes to a heat tub.
If a radiant ray such as an X-ray emitted from a sample enters into the absorber, a Joule heat is generated in the thermometer by that radiant ray, and heat escapes to the heat tub while transmitting through the membrane. By adjusting a balance between the Joule heat and the escaping heat, the thermometer comprising a material becoming the superconductor at a temperature below a liquid nitrogen temperature (absolute temperature 77 K) is retained at an operation point in a region called a superconducting transformation end that is a transition state between a superconducting state and a normal conducting state. As a result, a large resistance change occurs in the thermometer in regard to a minute temperature change, and the micro-calorie meter, by utilizing this, detects and analyzes the radiant ray.
The micro-calorie meter utilizing this superconducting transformation end of the superconductor is called a TES (Transition Edge Sensor).
FIG. 6 is a schematic constitution diagram of a conventional superconducting radiometry apparatus using the micro-calorie meter.
A micro-calorie meter 1 and an input coil 3 are connected, and to it there is connected in parallel a shunt resistance 2 having a resistance value sufficiently smaller than the micro-calorie meter 1, and a constant voltage is applied by a bias electric source 5.
An electric current flowing to the micro-calorie meter 1 is detected as an electric field signal by an SQUID amplifier 4, in which a superconducting quantum interference device (SQUID: Superconducting QUantum Interference Device) operating at the temperature below the liquid nitrogen temperature (absolute temperature 77 K) is plurally connected in series, and thereafter it is converted into an electric signal and amplified. And, an output signal from the SQUID amplifier 4 is sent to a room temperature amplifier 6, shaped and amplified. An output voltage from the room temperature amplifier 6 is selected while corresponding to a peak value by a wave height analyzer 7 for every energy to thereby be accumulated, and a result is spectrum-displayed by a display device (not shown in the drawing) or the like.
Here, if an X-ray which is one of the radiant rays consisting of, e.g., a corpuscular beam, an electromagnetic wave or the like is irradiated to the micro-calorie meter 1, a temperature of the thermometer minutely rises by the X-ray entering to the absorber of the micro-calorie meter 1. By it, a resistance value of the thermometer of the micro-calorie meter 1 increases. And, since the thermometer is retained at the constant voltage, the electric current flowing to the micro-calorie meter 1 decreases.
Here, since the electric current decreases, it acts in a direction in which the temperature of the thermometer lowers, and a negative feedback occurs so as to return it to a constant temperature. This is called a self electron-thermal feedback (ETF: Electron-Thermal Feedback). By optimizing the voltage of the bias electric source and a temperature of the heat tub, the thermometer of the micro-calorie meter is kept to the superconducting transformation end.
Furthermore, a displacement of the electric current flowing to the micro-calorie meter 1 is detected by the room temperature amplifier 6 through the SQUID amplifier or the like.
A peak value of an output signal from the room temperature amplifier 6 has a relation monotonously increasing in regard to an energy value of the entered X-ray. An energy of the X-ray which entered to the micro-calorie meter 1 can be found by previously finding a correlation diagram between a peak value of the output voltage from the room temperature amplifier 6 and an energy of the X-ray, and measuring a peak value of the generated output voltage.
Here, a relation between the energy (E) of the X-ray which entered to the micro-calorie meter 1 and a displacement quantity (ΔI) of the electric current is denoted by an expression 1. Here, Vn is an operating voltage, and τeff a time constant of an electric current pulse.E=ΔIVnτeff  Expression 1
By measuring this displacement quantity of the electric current, it is possible to find the energy of the entered X-ray (e.g., refer to K. D. Irwin, “An application of electrothermal feedback for high resolution cryogenic particle detection”, Applied Physics Letters, Volume 66, 1998-2000 (1995)).
However, in the superconducting radiometry apparatus like this, in a case where the energy of the entering X-ray is constant, although the peak value of the output signal from the room temperature amplifier becomes constant, there is such an issue that the peak value of the output signal from the room temperature amplifier changes by the fact that there occurs a thermal radiation following upon a temperature change of a thermal shield plate for thermally protecting the micro-calorie meter from an outside, or a resistance change of the micro-calorie meter, which follows upon a change in an external magnetic field exerted on the micro-calorie meter, so that a shift of the detected X-ray energy occurs.
Whereupon, in a case where the peak value of the output signal from the room temperature amplifier in regard to the X-ray of a fixed energy changes with a time, in order to obtain a high energy resolving power, it is necessary to perform an energy calibration so as not to change in regard to the time.
Whereupon, a conventional energy calibration method is explained by using FIG. 7 and FIG. 8.
(1) FIG. 7 is one example in which the output signal from the room temperature amplifier in regard to the time is measured by pulse-irradiating the X-ray whose energy value is already known to the micro-calorie meter.
(2) FIG. 8 is one in which there is plotted the output signal from the room temperature amplifier, which is measured for every one pulse in FIG. 7.
(3) A correction function in regard to the time is found by using a peak value 20 found from the plot of the output signal in FIG. 8. Here, a correction function 21 is found as a linear function.
(4) FIG. 9 is one example in which the energy calibration is performed by using the correction function 21 such that the peak value of the output voltage from the room temperature amplifier becomes constant in regard to the time.
Further, FIG. 10 is a schematic constitution diagram of other superconducting radiometry apparatus using the micro-calorie meter.
In regard to the superconducting radiometry apparatus in FIG. 6, which was explained before, a resistor 22 is additionally provided in parallel to the micro-calorie meter 1, and a feedback circuit 19 connected to the room temperature amplifier 6 is connected. And, although a resistance of the micro-calorie meter rises by the fact that the temperature of the micro-calorie meter 1 rises by the entered X-ray, the temperature of the micro-calorie meter 1 is returned to the origin by applying the electric current or the voltage to the resistor 22 from the feedback circuit on the basis of the electric current corresponding to the output signal from the room temperature amplifier 6. In other words, by making such that the resistance of the micro-calorie meter 1 early returns to an original operating point, a response speed can be improved (e.g., refer to JP-A-2002-236052 Gazette).
However, in the superconducting radiation detection apparatus described above, the following problems are encountered.
In the above energy calibration method, since the correction function is found and the correction of the peak value of the output signal is performed after the peak values of all output voltages are obtained by pulse-irradiating the X-ray, there is an issue that a processing time till the correction function is obtained becomes long. Further, there is also the fact that the radiation heat and the magnetic field from the outside, which become a cause of a fluctuation of the peak value of the output signal from the room temperature amplifier, fluctuate every moment, so that it is impossible to accurately perform the correction by a correction method in an off-line.
Alternatively, in the superconducting radiation detection apparatus in which there is provided the feedback circuit in order to return the resistance of the micro-calorie meter to the original level, there remains such an issue that, similarly as described above, when thermal radiation occurs following upon the temperature change of the thermal shield plate for thermally protecting the micro-calorie meter from the outside, or the resistance change of the micro-calorie meter, which follows upon the change in the external magnetic field exerted on the micro-calorie meter, there occurs a shift of the detected X-ray energy with the peak value of the output signal from the room temperature amplifier being changed intact only by rapidly returning the resistance of the micro-calorie meter to the origin under a state including the resistance change by performing the feedback to the resistor by the feedback circuit.